The present invention relates to adhesion promoters that improve adhesion between film-forming coatings and adhesives and polyolefinic substrates. The adhesion promoters are prepared by controlled radical polymerization using a halogenated polyolefin as a macroinitiator.
Polyolefins, such as polypropylene and polyethylene are used in a wide variety of molding applications including, for example, in preparation of molded parts for use in the automotive, industrial and appliance markets. The preparation of such molded articles generally includes the steps of molding an article from the polyolefin resin and applying to the molded article one or more film-forming coating layers to protect and/or color the article and/or an adhesive to attach the molded article to another article.
One difficulty with use of poyolefinic substrates is that typical film-forming coatings and adhesives do not adhere well to the substrate. In the case of a film-forming coating applied to the substrate, the layer delaminates. In the case of adhesives, adhesive failure is commonplace.
A solution to the failure of coatings and adhesives to adhere to the polyolefinic substrate is to include a layer of a film-forming composition including a chlorinated polyolefin (CPO) between the substrate and the film-forming coating or adhesive. This adds a processing step and, since chlorinated polyolefins are relatively expensive, adding to the cost of using polyolefins to produce molded parts.
U.S. Pat. No. 5,955,545 discloses use of CPO-acrylic graft copolymers as adhesion promoters that assertedly improve adhesion of subsequent coating layers and/or adhesives to polyolefins. However, these graft copolymer adhesion promoters are prepared by standard free radical polymerization methods and suffer from high polydispersity and the presence of non-graft polymer chains and acrylic copolymers and homopolymers in the resin composition as a result of the random nature of standard radical polymerization processes. The high polydispersity and additional non-graft chains present in the same mixture as the graft polymer results in incomplete or inefficient adhesion promotion and interference with the curing dynamics of the resin. Further, these compositions are unstable, readily falling out of solution, especially when incorporated in a film-forming composition, and they absorb and scatter light, giving a hazy appearance. Thus, they are unsuitable for many coating applications.
It is, therefore, desirable to have well defined adhesion promoting material that includes polyolefinic segments or portions that interact strongly with polyolefinic substrates as well as portions or non-polyolefinic segments that interact well with film-forming resins, crosslinkers and/or curing agents and solvents that are present in a typical coating composition. It is also desirable that the adhesion promoter be of a more defined architecture than is typically found in graft copolymers formed by a free radical process. The defined architecture will allow for design of copolymers that interact with other components of the coating composition in a consistent manner with less contamination with undesirable polymer species that typically result from free radical grafting to CPOs. The low polydispersity of the material, combined with the substantial absence of undesirable polymer species would yield a clear, stable coating additive or coating composition that would adhere well to polyolefinic substrates.
A wide variety of radically polymerizable monomers, such as methacrylate and acrylate monomers, are commercially available and can provide a wide range of properties including, for example, hydrophilic and hydrophobic properties, the ability to interact with crosslinkers, or to self crosslink.
U.S. Pat. Nos. 5,807,937; 5,789,487; and 5,763,548 and International Patent Publication Nos. WO 98/40415; WO 98/01480; WO 97/18247; and WO 96/30421 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The ""937 and ""548 patents also describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications including, for example, dispersants and surfactants.
U.S. Pat. Nos. 5,478,886; 5,272,201; 5,221,334; 5,219,945; 5,085,698; 4,812,517; and 4,755,563 describe ABC, AB and BAB block copolymers and pigmented ink compositions containing such block copolymers. The block copolymers of the ""886, ""201, ""334, 1945, ""698, ""517 and ""563 patents are described as being prepared by living or stepwise polymerization processes, such as anionic or group transfer polymerization.
A number of initiators and macroinitiators are known to support ATRP polymerization. These initiators are described, for example, in U.S. Pat. Nos. 5,807,937 and 5,986,015. U.S. Pat. No. 5,807,937 discloses a number of initiators, including a macroinitiator, where halide groups attached to an activated benzylic carbon serve as the initiating site. The ""937 patent discloses that benzyl halides can be efficient initiators for ATRP in monomeric form as well as in a polymer.
WO 9840415 A1 discloses ATRP macroinitiators having an activated halogen, which have been prepared by chlorosulfonation of polyethylene. The chlorosulfonyl group is known to be a good ATRP initiator in monomeric form.
Paik et al. (xe2x80x9cSynthesis and Characterization of Graft Copolymers of Poly (vinyl chloride) with Styrene and (Meth) acrylates by Atom Transfer Polymerizationxe2x80x9d, Macromol. Rapid Commun., 19, 47-52(1998)) disclose that polyvinyl chloride is incapable of serving as an initiator in an ATRP process. Paik further discloses that ATRP can be initiated by the activated chlorine in a chloroacetate group attached to a PVC backbone. Paik also discloses that the secondary chlorines on the PVC backbone do not initiate ATRP. Collectively, the prior art indicates that effective ATRP macroinitiators should contain activated halogens.
In accordance with the present invention, an alkenyl (co)polymer is provided that is prepared by polymerizing alkenyl monomers in the presence of an initiator or macroinitiator (collectively xe2x80x9can initiatorxe2x80x9d) having halide groups attached to tertiary carbons under atom transfer radical polymerization conditions. An example of such an initiator is, without limitation, a halogenated polyolefin such as a chlorinated or brominated polypropylene, polybutylene or branched polyethylene. This polymer finds use in a variety of applications, such as, without limitation, in compositions for coating, molding and extruding. A typical use for the copolymer is as an additive to a film-forming resin composition for coating a polyolefinic substrate. The additive both promotes interlayer adhesion between the coating composition layer and the polyolefinic substrate and can be crosslinked into the film-forming resin.
A curable film-forming composition including the alkenyl-halogenated polyolefin copolymer is also provided. The halogen content of the halogenated polyolefin embodiment is typically either chlorine or bromine. The number of halide groups in the initiator can vary, but typically falls between about 15% to 45% by weight of the initiator, as is commonly found in commercially available halogenated polyolefins.
A method of coating a polyolefinic substrate also is provided that includes applying to the polyolefinic substrate a film-forming composition comprising the above-described vinyl-halogenated polyolefin copolymer. A coated article prepared according to the method is also described.